SpaceX will make it a double when it launches its next Falcon 9 into the black from Space Launch Complex 4E (SLC-4E) at Vandenberg AFB in California on Tuesday, May 22. The “Block 4” Falcon 9, which flew previously for that top-secret ZUMA mission back in November, will carry two satellite payloads into Low Earth Polar Orbit– Iridium-6 and GRACE-FO.

The instantaneous launch window will open at 3:47 EST (19:47 UTC). The GRACE-FO satellites will deploy early in the mission (less than twelve minutes after liftoff). The five Iridium NEXT satellites will deploy roughly an hour into the mission.

As the name suggests, Iridium-6 will be the sixth mission SpaceX has flown for Iridium as it replaces and upgrades the world’s largest commercial satellite network. At the conclusion of this mission, SpaceX will have just two missions remaining for Iridium, after which it will have delivered a total of 75 new satellites into orbit– 66 operational satellites and nine on-orbit spares. The ongoing Iridium NEXT mission is one of the largest “tech upgrades” in space history.

Meanwhile, the NASA/German Research Centre for Geosciences (GFZ) GRACE Follow-On (GRACE-FO) mission is intended to continue GRACE’s 15-year legacy of tracking the movement of Earth’s mass. These twin satellites will allow the distribution of Earth’s mass to be mapped monthly and tracked over larger periods of time. This will, in turn, provide valuable data about climate change, including changes in ice sheets and glaciers, water levels in large lakes and rivers as well as sea level, and various water and energy cycles.

Since the Falcon 9 being used for this mission is a “Block 4” model, it is unlikely that there will be a landing of the rocket’s first stage. SpaceX will attempt to capture the fairings, however, and Mr. Steven, the company’s “spider-boat” that was designed especially for that purpose, is already at sea.